A computer network is a geographically distributed collection of interconnected subnetworks for transporting data between stations, such as computers. A local area network (LAN) is an example of such a subnetwork consisting of a transmission medium, such as coaxial cable or twisted pair, that facilitates relatively short-distance communication among interconnected computers or "stations." The stations typically communicate by exchanging discrete packets or frames of data according to predefined protocols. In this context, a protocol denotes a set of rules defining how the stations interact with each other.
Most networks are organized as a series of hardware and software levels or "layers" within each station. These layers interact to format data for transfer between, e.g., a source station and a destination station communicating over the network. Specifically, predetermined services are performed on the data as it passes through each layer and the layers communicate with each other by means of the predefined protocols. This layered design permits each layer to offer selected services to other layers using a standardized interface that shields the other layers from the details of actual implementation of the services.
In an attempt to standardize network architectures, i.e., the sets of layers and protocols used within a network, a generalized model has been proposed by the International Standards Organization (ISO). The model, called the Open Systems Interconnection (OSI) reference model, is directed to the interconnection of systems that are "open" for communication with other systems. The proposed OSI model has seven layers which are termed, in ascending interfacing order, the physical, data link, network, transport, session, presentation, and application layers. These layers are arranged to form a "protocol stack" in each station of the network.
FIG. 1 illustrates a schematic block diagram of conventional protocol stacks 125 and 175 used to transmit data between a source station 110 and a destination station 150, respectively, of a LAN 100. Each protocol stack comprises a collection of protocols, one per layer, and is preferably structured according to the OSI seven-layer model. As can be seen, the protocol stacks 125 and 175 are physically connected through a communications channel 180 at the physical layers 124 and 164. For ease of description, the protocol stack 125 will be described.
In general, the application layer 12 contains a variety of protocol functions that are commonly needed by software processes, e.g., sending process 104, executing on the station, while the presentation layer 114 is responsible for the presentation of transmitted data in a meaningful manner to the application layer. The session layer 116, transport layer 118 and the network layer 120 are substantially involved in providing pre-defined sets of services to aid in connecting the source station to the destination station.
IEEE standard 802 defines a flexible network architecture oriented to the implementation of LANs. Although it generally conforms with the OSI model, the IEEE approach addresses only the lowest two layers of that model, the physical and data link layers. Specifically, the physical layer 124 is concerned with the actual transmission of signals across the communication channel; in this context, the physical layer defines the types of cabling, plugs and connectors used in connection with the channel.
The data link layer 122, on the other hand, is responsible for transmission of data from one station to another. In the IEEE 802 architecture, the data link layer is divided into two sublayers: logical link control (LLC) and media access control (MAC). The LLC sublayer 180 allows the overlying network layer to access the services of the LAN without regard to the actual network implementation; more specifically, the LLC layer initiates control signal interchange, organizes data flow, interprets commands and generates responses.
The MAC sublayer 182 is primarily concerned with controlling access to the transmission medium and, to that end, defines rules or procedures by which the stations must abide in order to share the medium. The MAC layer further provides addressing and framing functions, the latter including the addition of header and trailer information needed to identify the boundaries of frames and to synchronize communication between source and destination stations.
Data transmission over LAN 100 therefore consists of generating data in, e.g., sending process 104 executing on the source station 110, passing that data to the application layer 112 and down through the layers of the protocol stack 125, where the data are sequentially formatted as a frame for delivery onto the channel 180 as bits. Those frame bits are then transmitted to the protocol stack 175 of the destination station 150, where they are passed up that stack to a receiving process 174. Data flow is schematically illustrated by solid arrows.
Although actual data transmission occurs vertically through the stacks, each layer is programmed as though such transmission were horizontal. That is, each layer in the source station 100 is programmed to transmit data to its corresponding layer in the destination station 150, as schematically shown by dotted arrows. To achieve this effect, each layer of the protocol stack 125 in the source station 110 typically adds information (in the form of a header field) to the data frame generated by the sending process as the frame descends the stack. At the destination station 150, the various encapsulated headers are stripped off one-by-one as the frame propagates up the layers of stack 175 until it arrives at the receiving process.
As noted, a significant function of each layer in the OSI model is to provide services to the other layers. One type of service offered by the layers is a "connectionless" transmission service where each transmitted packet carries the full address of its destination through the network. A key function of a routing device such as a bridge or a router is determining the next LAN or station to which the packet is sent. A bridge operates at the data-link level, connecting one or more LANs together (that is, facilitating the transfer of messages among the LANs connected to the bridge). A router operates at the network level and may span clusters of LANs. When the network layer receives a packet from the transport layer for transmission over the network, it encapsulates the packet with a header containing, inter alia, source and destination addresses. An example of a network layer protocol is the Internet (IP) network layer protocol.
Within a LAN, access is typically shared among the stations in accordance with various control methods depending upon the topology of the subnetwork and the type of transmission control employed. A popular subnetwork topology is a ring network that may be formed by configuring the communication channel as a loop and coupling the stations at intervals around the loop. The stations communicate by transmitting and receiving discrete signals in the form of data frames according to predefined protocols. Acceptance of a frame by each station, in turn, is determined on the basis of an address contained in the frame.
With LANs that employ a ring topology, a commonly used access control method is token passing. Token passing is a form of distributed transmission control wherein all the stations on the subnetwork cooperate in controlling access to the communication channel. Here, a small message or "token" is passed from one station to the next along the ring. If the token is marked as free, the station receiving it can transmit a message over the ring. A token ring network is an example of a ring topology that uses token passing as an access control method.
Token ring networks may be interconnected using routers and bridges that transfer frames between the rings. A route is the path--that is, the sequence of routing devices and intermediate LANs--a frame travels from a source station to a destination station, and is specified in the header of the frame. Frequently, a message can reach its destination over more than one path. In "source route bridging," it is the source station that typically determines the particular path the message is to follow, and writes that path into the header. Source-route bridges usually are also configured to handle frames lacking specific routes (i.e., "transparent" frames); such bridges are referred to as SR-TB bridges, and in these cases, the first bridge to receive the frame determines the path.
Unless the source or routing device has previously sent messages to the destination station, it generally must undertake a "route-discovery procedure" to identify the sole or optimal path the message is to travel. Once the source station has "discovered" this path to the destination, it caches the path for future use, and then transmits frames specifying the path and the address of the destination station onto the network.
In one common route-discovery procedure, the source station issues the frame as an "all-paths explorer" packet that is received by each station on the local ring subnetwork. Each routing device copies the frame and supplies information relating to the route (i.e., bridge and ring numbers) within its RI field; it then distributes the copy to all stations on its interconnected ring. Eventually, a copy of the broadcast explorer frame reaches every station on every LAN of the network. Each station may respond to the source by issuing a response frame containing its MAC address and the routing information. The source station examines the information contained in these response frames and selects (e.g., based on the round-trip time for return) a path to the intended destination. Routing devices along the path may also cache the path to the destination station.
In many instances, and depending on the network topology, the path the frame follows may not be determined by routing information contained in the frame itself. An example of such a topology involves "data link switching," or DLSw, which provides a forwarding mechanism across wide-area links. In traditional bridging, the data link connection is end-to-end, i.e., effectively continuous between communicating end stations; a frame originating on a source LAN traverses one or more bridges specified in the path over the LLC connection to the destination LAN. In a system implementing DLSw, by contrast, the LLC connection terminates at the first DLSw bridge or router. The DLSw device multiplexes the LLC connections onto a transport connection (usually a "transport-control protocol," or TCP connection) to another DLSw bridge or router. In this way, the individual LLC connections do not cross a wide-area network, thereby reducing traffic across this network; the LLC connections from the source LAN to the transmitting data link switch, and from the receiving data link switch to the destination LAN, are entirely independent from one another. Data link switching may be implemented on multi-protocol routers capable of handling DLSw as well as conventional (e.g., SRB) frames. The DLSw forwarding mechanism is well-known and described in detail in Wells et al., Request for Comment (RFC) 1795 (1995).
Because DLSw devices communicate with one another relatively autonomously (so that, for example, a receiving switch may be selected from among several candidates according to load-balancing or other network considerations), the ultimate routing path cannot be established at the source. Instead, each data link switch will build a set of tables listing the identities of each other switch capable of reaching a specific MAC address. As a consequence, search traffic is kept to a minimum, but expansion of the network imposes an increasing storage burden on DLSw devices.
Indeed, as the size of the network grows, the cache of paths and addresses each device accumulates to facilitate exchange of messages with other stations can increase exponentially. Even in modestly sized networks, this can require substantial storage capacity that raises costs and decreases performance, as routing times increase with the size of the path table. Moreover, because stations can be swapped among LANs and entire LANs eliminated or rerouted, the paths must be updated or removed relatively frequently. Once again, these operations become burdensome as network size increases.